Is there any possibility for one token (e.g. Augur's REP) to become more valuable than the total Ether market cap? If so, how would that be possible?
In Security of PoS if/when Assets (Tracked or Native) to Ethereum exceed the total Value of Ether, the security of the asset(s) whose total value is greater than Ethereum's is discussed, but not the mechanism through which such a scenario could arise.
Tokens are just assets on top of the ethereum blockchain. They are not related to the underlying network consensus. The staking happens only with ETH. Thus, for your outlined attack vector you need only Ether.
But that said, I want to highlight that the Casper proof of stake is way different than other known implementations.
See also other questions tagged with casper here or view the interview with Vlad Zamfir on Ethereum PoS here.
